IMS reregistration method and system therefor

ABSTRACT

A method and system in which upon occurrence of a P-CSCF failure, a network sends an IMS reregistration request message to a UE. An S-CSCF or an AS monitors a P-CSCF to detect occurrence of a failure in the P-CSCF. Upon detecting the occurrence of a P-CSCF failure, the S-CSCF or the AS sends a request message for IMS reregistration to the UEs being served by the failed P-CSCF. In this manner, the UE can continuously receive the service even though a failure occurs in the P-CSCF.

More than one Reissue Application has been filed for U.S. Pat. No.8,018,858. This application is a Continuation Reissue of U.S.application Ser. No. 14/026,632, which is a Reissue Application of U.S.Pat. No. 8,018,858.

PRIORITY

This application claims priority under 35 U.S.C. §119(a) to a KoreanPatent Application filed in the Korean Intellectual Property Office onJan. 11, 2007 and assigned Serial No. 2007-3534, and a Korean PatentApplication filed in the Korean Intellectual Property Office on Mar. 19,2007 and assigned Serial No. 2007-26651, the contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a network system including aCircuit Switched (CS) domain, an Internet Protocol (IP) MultimediaSubsystem (IMS) domain and a Packet Switched (PS) domain, and inparticular, to a method and system in which a network sends an IMSreregistration request message to a User Equipment (UE) upon occurrenceof a Proxy Call Session Control Function (P-CSCF) failure.

2. Description of the Related Art

A CS scheme, the typical scheme for voice services and real-timeservices, means a scheme in which a CS-based fixed call route isestablished between one user and its counterpart user. IMS is suitablefor data services, messaging services and file transfer services, as itimproves transmission efficiency and guarantees transmission stabilitywith the use of a PS-based unfixed call route based on Internet Protocol(IP). Such an IMS can support not only the simple point-to-point callbut also the point-to-multipoint call in which multiple userssimultaneously take part. In the CS scheme, transmission of messages anduser traffics is achieved over a CS call, and in IMS, the transmissionof messages and user traffics is achieved over an IMS session.

FIG. 1 illustrates a configuration of a network to which the presentinvention is applicable.

Referring to FIG. 1 , the network, to which the present invention isapplicable, is roughly divided into a radio access network 106 and acore network 102. A User Equipment (UE) 104 accesses the core network102 over the radio access network 106. The core network 102 is roughlydivided into a CS domain 112, a PS domain 110, and an IMS domain 108.

The CS domain 112 is composed of entities supporting a CS call. Amongthe entities, a Mobile Switching Center (MSC; not shown) processes thecall signals incoming and outgoing from/to a mobile base station, andperforms a centralized control function so that the mobile base stationcan efficiently operate. A Media Gateway (MGW; not shown) is a gatewaysupporting the bearer traffics and control traffics. A Short MessageService (SMS) Service Center (SMS-SC) 124 serves to transfer SMS, or SMSmessages, to users. A Visitor Location Register (VLR) 122 manages allthe user data necessary for user call processing and mobilitymanagement.

The PS domain 110 supports PS-based services, and is roughly dividedinto a Serving GPRS Support Node (SGSN) and a Gateway GPRS Support Node(GGSN).

A Home Subscriber Server (HSS) 120 or a Home Location Register (HLR),which is an entity used for both the CS domain 112 and the PS domain110, manages subscription information, service information, and locationinformation of the users. The HSS 120 is an evolved form of the HLR, andeither of the two entities can be used in the present invention.

The IMS domain 108 is composed of entities supporting the IMS session.Among the entities, a Proxy-Call Session Control Function (P-CSCF) 116,a Serving-Call Session Control Function (S-CSCF) 114, and an ApplicationServer (AS) 118 are related to the present invention. The P-CSCF 116 isan entry point for the IMS domain 108 of the UE 104, and with the usethereof, the UE 104, when accessing the radio access network 106,receives a P-CSCF address from the radio access network 106 or acquiresit through a Dynamic Host Configuration Protocol (DHCP) query. TheS-CSCF 114, an entity for performing the most important function ofestablishing an IMS session, serves to receive and process a SessionInitiation Protocol (SIP) message delivered from the UE 104. That is, toprovide the service as requested by the UE 104, the S-CSCF 114 deliversSIP messages to the entities in the IMS domain 108 so that the sessionis completed. The AS 118 is an entity for providing applications to theusers, like an E-mail server.

In the network with the foregoing configuration, the UE should firstperform IMS registration in order to receive IMS services. Through thisprocess, a P-CSCF and an S-CSCF that serve the UE are determined.However, when it is impossible to serve the UE as failure occurs in theP-CSCF, the UE cannot receive the IMS service even though the radiobearer is alive between the UE and the radio access network. Forexample, if an incoming call for the UE arrives when P-CSCF failure hasoccurred, the network cannot deliver the call to the UE.

Conventionally, upon occurrence of the P-CSCF failure, the UE perceivesthe need for IMS reregistration and performs it as follows. The UEperforms IMS registration by transmitting ‘REGISTER’, which is one ofthe SIP Methods, to an IMS network. Thereafter, the UE refreshes the IMSregistration by sending REGISTER to the IMS network at stated intervals.After sending REGISTER, if a response message thereto is not received,the UE can perceive the occurrence of the P-CSCF failure. However, whenthe time interval for which the REGISTER is sent is long and the P-CSCFfailure has occurred immediately after the refreshment of the IMSregistration, the user cannot receive the call being delivered theretofor quite a long time.

SUMMARY OF THE INVENTION

An aspect of the present invention is to address at least the problemsand/or disadvantages and to provide at least the advantages describedbelow. Accordingly, an aspect of the present invention provides anetwork entity for monitoring a P-CSCF when a user cannot receive a calldue to a P-CSCF failure, and a method and system for performing IMSreregistration using the same.

According to one aspect of the present invention, there is provided amethod for performing Internet Protocol (IP) Multimedia Subsystem (IMS)reregistration of a User Equipment (UE). The method includes assigningone of a plurality of Proxy-Call Session Control Functions (P-CSCFs) toa UE that has requested IMS registration; monitoring occurrence of afailure in the multiple P-CSCFs; and upon detecting occurrence of afailure from at least one of the multiple P-CSCFs as a result of themonitoring, notifying at least one of the UEs being served by the failedP-CSCF to perform IMS reregistration.

According to another aspect of the present invention, there is provideda method for performing Internet Protocol (IP) Multimedia Subsystem(IMS) reregistration by a User Equipment (UE). The method includesreceiving assignment of one of a plurality of Proxy-Call Session ControlFunctions (P-CSCFs) through IMS registration; when a failure occurs inthe assigned P-CSCF, receiving an IMS reregistration request messagefrom an Application Server (AS); and sending a request for IMSreregistration to a Serving-Call Session Control Function (S-CSCF), tobe assigned a new P-CSCF.

According to further another aspect of the present invention, there isprovided a system for supporting Internet Protocol (IP) MultimediaSubsystem (IMS) reregistration of a User Equipment (UE). The systemincludes a Serving-Call Session Control Function (S-CSCF) for monitoringa Proxy-Call Session Control Function (P-CSCF) that serves the UE, todetect occurrence of a failure of the P-CSCF; and an Application Server(AS) for receiving, from the S-CSCF, a failure occurrence notificationmessage of the P-CSCF and information on UEs being served by the failedP-CSCF, and sending an IMS reregistration request message to the UEs.

According to yet another aspect of the present invention, there isprovided a system for supporting Internet Protocol (IP) MultimediaSubsystem (IMS) reregistration of a User Equipment (UE). The systemincludes a Proxy-Call Session Control Function (P-CSCF) for serving a UEthat has performed 3^(rd) party registration; and an Application Server(AS) for, upon detecting occurrence of a failure of the P-CSCF, sendingan IMS reregistration request message to UEs being served by the failedP-CSCF.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a diagram illustrating a configuration of a network to whichthe present invention is applicable;

FIG. 2A is a diagram illustrating an operation of S-CSCF when the S-CSCFmonitors a P-CSCF according to a first embodiment of the presentinvention;

FIG. 2B is a diagram illustrating an operation of a MSG-AS when theS-CSCF monitors a P-CSCF according to the first embodiment of thepresent invention;

FIG. 2C is a diagram illustrating an operation of UE when the S-CSCFmonitors a P-CSCF according to the first embodiment of the presentinvention;

FIG. 3A is a diagram illustrating an operation of UE when the ASmonitors a P-CSCF according to a second embodiment of the presentinvention;

FIG. 3B is a diagram illustrating an operation of an MON-AS when the ASmonitors a P-CSCF according to the second embodiment of the presentinvention;

FIG. 4 is a diagram illustrating a control message flow according to thefirst embodiment of the present invention;

FIG. 5 is a diagram illustrating a control message flow according to thesecond embodiment of the present invention; and

FIG. 6 is a diagram illustrating an operation of an SMS-AS according tothe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be described indetail with reference to the accompanying drawings. In the followingdescription, a detailed description of known functions andconfigurations incorporated herein has been omitted for clarity andconciseness.

The present invention monitors a P-CSCF to send a request message forIMS reregistration to the UEs being served by the failed P-CSCF when theuser cannot receive a call due to a P-CSCF failure, thereby allowing theUE receiving the request message to perform IMS reregistration.

First Embodiment

According to a first embodiment of the present invention, when a failureoccurs in a P-CSCF, an S-CSCF delivers, to an AS, a list of andinformation on the UEs being served by the P-CSCF. Then the AS deliversa request message for IMS reregistration to the UEs, and upon receivingthe request message, the UEs perform IMS reregistration.

FIGS. 2A to 2C illustrate operations of S-CSCF, AS and UE, respectively,when the S-CSCF monitors a P-CSCF according to the first embodiment ofthe present invention.

In the first embodiment of the present invention, the AS serves toreceive information on the UEs being served by the failed P-CSCF and todeliver an IMS reregistration request message to the UEs. The AS in thefirst embodiment of the present invention will be referred to as‘MSG-AS’ to distinguish it from an AS in a second embodiment of thepresent invention. However, it should be understood that it is notintended to limit the scope of the present invention to the name of theentities.

FIG. 2A illustrates an operation of an S-CSCF according to the firstembodiment of the present invention.

Referring to FIG. 2A, the S-CSCF monitors the P-CSCF in step 200. In anexemplary method for monitoring the P-CSCF, the S-CSCF periodicallysends a signal to the P-CSCF and interprets a state of the P-CSCFdepending on a response signal received in response thereto. The inverseP-CSCF monitoring method is also available. However, other monitoringmethods can also be used in the present invention. While monitoring theP-CSCF, if a P-CSCF failure occurs in step 202, the S-CSCF proceeds tostep 204 where it delivers, to the MSG-AS, information on the UEs beingserved by the failed P-CSCF. It will be assumed herein that as theS-CSCF and the MSG-AS belong to the same service provider, or have madea particular agreement, so that there is no security problem even thoughthe information on the UEs managed by the S-CSCF is delivered to theMSG-AS. Occasionally, this assumption may be unnecessary. When P-CSCFfailure has not occurred in step 202, the S-CSCF returns to step 200where it continuously monitors the P-CSCF.

FIG. 2B illustrates an operation of an MSG-AS according to the firstembodiment of the present invention.

Referring to FIG. 2B, in step 210, the MSG-AS receives a messagenotifying the occurrence of a failure in the P-CSCF. In step 212, theMSG-AS receives information on the UEs being served by the failedP-CSCF, from the S-CSCF. The message received in step 210 can beincluded in the message received in step 212, and the two processes canbe performed independently. Upon receiving the information on the UEs,the MSG-AS sends a request message for IMS reregistration to thecorresponding UEs using the received information in step 214. The MSG-ASsends the IMS reregistration request message to an SMS-AS, which is anentity for interworking SMS with IP, and a detailed description of themethod in which the UE receives the IMS reregistration request messageusing SMS will be made below with reference to FIG. 4 .

FIG. 2C illustrates an operation of a UE according to the firstembodiment of the present invention.

Referring to FIG. 2C, upon receiving an IMS reregistration requestmessage from the MSG-AS in step 216, the UE performs IMS reregistrationin a new P-CSCF in step 218. To find a new P-CSCF, the UE can use theconventional method used in IMS.

Second Embodiment

According to a second embodiment of the present invention, an ASmonitors a P-CSCF serving the UE that has performed 3^(rd) partyregistration during IMS registration, to send a request message for IMSreregistration to the corresponding UEs upon occurrence of P-CSCFfailure. The AS in the second embodiment of the present invention willbe referred to as ‘MON-AS’ to distinguish it from the AS in the firstembodiment. However, it should be understood that it is not intended tolimit the scope of the present invention to the name of the entities.

FIGS. 3A and 3B illustrate operations of UE and AS, respectively, whenthe AS monitors a P-CSCF according to the second embodiment of thepresent invention.

FIG. 3A illustrates an operation of a UE according to the secondembodiment of the present invention.

Referring to FIG. 3A, in step 300, the UE performs 3^(rd) partyregistration in an MON-AS. The 3^(rd) party registration that the UEperforms during IMS registration means an operation in which the UEregisters its own information in the AS from which it desires to receivea service. Specifically, if the UE sends a REGISTER Method used forperforming IMS registration to the IMS network, the P-CSCF receives theREGISTER Method and forwards it to the S-CSCF that serves the UE. TheS-CSCF perceives the need for registration in the AS, depending on aninitial Filter Criteria (iFC) including service-related information ofthe UE. The S-CSCF forwards the REGISTER Method to the AS, and the ASregisters therein LE information necessary for its service provision.Thereafter, the UE receives a request message for IMS reregistrationfrom the MON-AS in step 302, and performs IMS reregistration in a newP-CSCF in step 304.

FIG. 3B illustrates an operation of an MON-AS according to the secondembodiment of the present invention.

Referring to FIG. 3B, the MON-AS receives 3^(rd) party registrationinformation from a UE in step 306. In this process, the MON-AS storesinformation on the UE and a P-CSCF serving the UE. Thereafter, theMON-AS monitors the P-CSCF in step 308. The P-CSCF monitoring method hasbeen described in FIGS. 2A to 2C. In step 310, the MON-AS determineswhether any failure has occurred in the P-CSCF. If it is determined thatany failure has occurred in the P-CSCF, the MON-AS sends in step 312 arequest message for IMS reregistration to the UEs being served by thefailed P-CSCF among the UEs that have performed 3^(rd) partyregistration. However, if it is determined in step 310 that P-CSCFfailure has not occurred, the MON-AS returns to step 308 where itcontinuously performs P-CSCF monitoring.

FIG. 4 illustrates a control message flow according to the firstembodiment of the present invention.

Referring to FIG. 4 , a P-CSCF1 402 is an entity in which a UE 400 hasperformed IMS registration, and a P-CSCF2 404 is an entity in which theUE 400 will perform reregistration upon receiving an IMS reregistrationmessage. An SMS-AS 410, an entity for interworking SMS with IP, servesto send an IP message in an SMS form to the UE 400 in association withan SMS Service Center (SMS-SC) 412. A description of the other entitiesdescribed above is omitted for conciseness.

As described in FIGS. 2A to 2C, an S-CSCF 406 monitors the P-CSCF1 402in step 414. Upon detecting a failure of the P-CSCF1 402 in step 416,the S-CSCF 406 notifies an MSG-AS 408 of the failure of the P-CSCF1 402in step 418. Further, in step 420, the S-CSCF 406 transmits informationon the UEs being served by the failed P-CSCF1 402 to the MSG-AS 408.Steps 416 and 418 follow the description given in FIGS. 2A to 2C. Insteps 422 and 424, the MSG-AS 408 transmits the information indicatingthe need for IMS reregistration to the UEs being served by the P-CSCF1402. To perform this process, the MSG-AS 408 can generate as many IPmessages as the number of UEs, and deliver the IP messages targeting theUEs to the SMS-AS 410. Alternatively, if the MSG-AS 408 delivers to theSMS-AS 410 a list of the UEs from which it will receive reregistrationmessages, the SMS-AS 410 can generate SMS and deliver it to each of theUEs. Upon receiving the IP message with the information indicating theneed for IMS reregistration from the SMS-AS 410 in step 424, the SMS-SC412 converts the IP message into the SMS format, and sends the SMSmessage to the UE 400 in step 428. Upon receiving the SMS requestmessage for IMS reregistration, the UE 400, to perform IMSreregistration, performs in step 428 a P-CSCF discovery process definedin the conventional IMS, and performs in step 432 IMS reregistration inthe new P-CSCF2 404. The succeeding IMS reregistration process followsthe conventional IMS registration process.

FIG. 5 illustrates a control message flow according to the secondembodiment of the present invention.

Referring to FIG. 5 , in steps 514 to 526, a UE 500 performs 3^(rd)party registration in an MON-AS 508. During the 3^(rd) partyregistration, the MON-AS 508 registers therein information on the UE 500that performs the registration, and information on a P-CSCF1 502 servingthe UE 500. Thereafter, the MON-AS 508 monitors the P-CSCF1 502 in step528. Upon detecting the occurrence of a failure of the P-CSCF1 502 instep 530, the MON-AS 508 sends, in steps 532 to 542, an IMSreregistration request message to the UE 500 being served by the P-CSCF1502 depending on the information on the UE 500 and the P-CSCF1 502,previously stored therein. Steps 532 to 542 are the same as steps 422 to432 of FIG. 4 , thus a description thereof is omitted for conciseness.

Next, with reference to the accompanying drawing, a description will bemade of an operation of the SMS-AS according to an embodiment of thepresent invention.

The SMS-AS receives, from the MSG-AS or the MON-AS, indicationinformation indicating the request for IMS reregistration for the UEsbeing served by the failed P-CSCF, and information on the UEs to whichit should transfer the indication information.

The indication information can be delivered to the LE(s) in (1) a methodof delivering the indication information using an IP signaling packet,(2) a method of delivering the indication information using a textincluded in a content body in an SIP message, and/or (3) a method ofgenerating an SMS message with the indication information, including theentire SMS message in a content body in an SIP message and deliveringthe resulting SIP message. Other delivery methods are also available forthe present invention.

FIG. 6 illustrates an operation of an SMS-AS according to an embodimentof the present invention.

Referring to FIG. 6 , the SMS-AS receives a message in step 600. In step601, the SMS-AS determines whether IMS reregistration request indicationand UE information are included in the received message. If the receivedmessage does not include the above information, the SMS-AS returns tostep 600 and receives a new message. However, if the received messageincludes the above information, the SMS-AS proceeds to step 602 where itdetermines whether the received message is an SIP message. If thereceived message is an SIP message, the SMS-AS proceeds to step 603where it checks a content type of the SIP message to determine whetherthe message included in the content body is an encapsulated type of theSMS message. If the content type of the SIP message indicates anencapsulated type of the SMS message, the SMS-AS proceeds to step 606where it extracts SMS included in the content body of the SIP message.Thereafter, in step 608, the SMS-AS delivers the extracted SMS to theUE.

However, if it is determined in step 602 that the received message isnot an SIP message, the SMS-AS proceeds to step 604 where it extractsIMS reregistration request indication and UE information included in thereceived message. In step 607, the SMS-AS generates an SMS to bedelivered to the UE, based on the information extracted in step 604.Thereafter, in step 608, the SMS-AS delivers the generated SMS to theUE.

However, if it is determined in step 603 that the content type of thereceived SIP message does not indicate an encapsulated type of the SMSmessage, the SMS-AS proceeds to step 605 where it extracts IMSreregistration request indication and UE information included in thecontent body of the SIP message. Thereafter, the SMS-AS generates an SMSto be delivered to each UE based on the extracted information in step607, and then transmits the generated SMS to each UE in step 608.

As is apparent from the foregoing description, according to the presentinvention, the user can continuously receive the service even though afailure occurs in the P-CSCF, and the service provider can rapidlydiscover the P-CSCF failure and take an appropriate action, therebyreducing the possible loss occurring due to the interruption of theservice provision.

In addition, the control message described herein can be realized inseveral message types such as SIP MESSAGE Method. The control message issubject to change in terms of its transmission order according to thecircumstances. In addition, the SMS used herein to send the IMSreregistration message to the UE can be realized with an UnstructuredSupplementary Service Data (USSD) message delivery scheme and/or othermessage delivery schemes.

While the invention has been shown and described with reference to acertain preferred embodiment thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A method for performing Internet Protocol (IP)Multimedia Subsystem (IMS) reregistration of a User Equipment (UE), themethod comprising: assigning one of a plurality of Proxy-Call SessionControl Functions (P-CSCFs) to a UE that has requested IMS registration;monitoring occurrence of a failure in the multiple P-CSCFs; and upondetecting occurrence of a failure from at least one of the multipleP-CSCFs as a result of the monitoring, notifying at least one of UEsbeing served by the failed P-CSCF to perform IMS reregistration.
 2. Themethod of claim 1, wherein the monitoring comprises: monitoring theP-CSCF by a Serving-Call Session Control Function (S-CSCF).
 3. Themethod of claim 2, wherein the notifying comprises: upon detecting theoccurrence of a failure of the P-CSCF, sending by the S-CSCF a failureoccurrence notification message to an Application Server (AS);transferring, by the S-CSCF, information on UEs being served by thefailed P-CSCF to the AS; and sending, by the AS, an IMS reregistrationrequest message to the UEs.
 4. The method of claim 3, wherein thetransferring information on UEs comprises: generating, by the AS, asmany IMS reregistration request messages as a number of UEs being servedby the failed P-CSCF, and sending the IMS reregistration requestmessages to a message server (SMS-AS); and converting, by the SMS-AS,the IMS reregistration request message into a Short Message Service(SMS) format, and sending the SMS to the UEs.
 5. The method of claim 3,wherein the transferring information on UEs comprises: generating, bythe AS, an IMS reregistration request message, and delivering thegenerated IMS reregistration request message to an SMS-AS along with alist of UEs that will receive the IMS reregistration request message;and converting, by the SMS-AS, the IMS reregistration request messageinto an SMS format, and delivering the SMS to the UEs.
 6. The method ofclaim 1, further comprising: before the monitoring, registering, by anAS, the P-CSCF serving a UE that has performed 3^(rd) partyregistration; wherein the monitoring comprises monitoring the P-CSCF bythe AS.
 7. The method of claim 6, wherein the notifying comprises:sending, by the AS, an IMS reregistration request message to the UEsbeing served by the failed P-CSCF.
 8. The method of claim 7, wherein thenotifying comprises: generating, by the AS, as many IMS reregistrationrequest messages as a number of UEs being served by the failed P-CSCF,and delivering the generated IMS reregistration request message to anSMS-AS; and converting, by the SMS-AS, the IMS reregistration requestmessage into an SMS format, and sending the SMS to the UEs.
 9. Themethod of claim 7, wherein the notifying comprises: generating, by theAS, an IMS reregistration request message, and delivering the generatedIMS reregistration request message to an SMS-AS along with a list of UEsthat will receive the IMS reregistration request message; andconverting, by the SMS-AS, the IMS reregistration request message intoan SMS format, and delivering the SMS to the UEs.
 10. A method forperforming Internet Protocol (IP) Multimedia Subsystem (IMS)reregistration by a User Equipment (UE), the method comprising:receiving assignment of one of a plurality of Proxy-Call Session ControlFunctions (P-CSCFs) through IMS registration; when a failure occurs inthe assigned P-CSCF, receiving an IMS reregistration request messagefrom an Application Server (AS); and sending a request for IMSreregistration to a Serving-Call Session Control Function (S-CSCF), tobe assigned a new P-CSCF.
 11. The method of claim 10, wherein the IMSreregistration request message is a Short Message Service (SMS) message.12. The method of claim 10, wherein the IMS reregistration requestmessage is an Unstructured Supplementary Service Data (USSD) message.13. A system for supporting Internet Protocol (IP) Multimedia Subsystem(IMS) reregistration of a User Equipment (UE), the system comprising: aServing-Call Session Control Function (S-CSCF) for monitoring aProxy-Call Session Control Function (P-CSCF) that serves the UE, todetect occurrence of a failure of the P-CSCF; and an Application Server(AS) for receiving, from the S-CSCF, a failure occurrence notificationmessage of the P-CSCF and information on UEs being served by the failedP-CSCF, and sending an IMS reregistration request message to the UEs.14. The system of claim 13, wherein the AS generates as many IMSreregistration request messages as a number of UEs being served by thefailed P-CSCF; wherein the system further comprises a message server(SMS-AS) for receiving the IMS reregistration request message from theAS, converting the received IMS reregistration request message into aShort Message Service (SMS) format, and sending the SMS to the UEs. 15.The system of claim 13, wherein the AS generates an IMS reregistrationrequest message; wherein the system further comprises an SMS-AS forreceiving the IMS reregistration request message from the AS along witha list of UEs that will receive the IMS reregistration request message,converting the IMS reregistration request message into an SMS format,and sending the SMS to the UEs.
 16. A system for supporting InternetProtocol (IP) Multimedia Subsystem (IMS) reregistration of a UserEquipment (UE), the system comprising: a Proxy-Call Session ControlFunction (P-CSCF) for serving a UE that has performed 3^(rd) partyregistration; and an Application Server (AS) for, upon detectingoccurrence of a failure of the P-CSCF, sending an IMS reregistrationrequest message to UEs being served by the failed P-CSCF.
 17. The systemof claim 16, wherein upon receiving the IMS reregistration requestmessage, the UE performs a P-CSCF discovery process and performs IMSreregistration.
 18. The system of claim 16, wherein the AS generates asmany IMS reregistration request message as a number of UEs being servedby the failed P-CSCF; wherein the system further comprises a messageserver (SMS-AS) for receiving the IMS reregistration request messagefrom the AS, converting the received IMS reregistration request messageinto an Short Message Service (SMS) format, and sending the SMS to theUEs.
 19. The system of claim 16, wherein the AS generates an IMSreregistration request message; wherein the system further comprises anSMS-AS for receiving the IMS reregistration request message from the ASalong with a list of UEs that will receive the IMS reregistrationrequest message, converting the IMS reregistration request message intoan SMS format, and sending the SMS to the UEs.
 20. A method forsupporting an internet protocol (IP) multimedia subsystem (IMS)registration of a user equipment (UE), the method comprising:transmitting, by a first network entity to the UE, information on anaddress of a proxy-call session control function (P-CSCF) based on adynamic host configuration protocol (DHCP); receiving, by the firstnetwork entity from a second network entity, a first message includingthe address of the P-CSCF for which the UE performed an initial IMSregistration based on the information on the address of the P-CSCF, andstoring the address of the P-CSCF, wherein the second network entity andthe P-CSCF are not the same network entity; transmitting, by the firstnetwork entity to the second network entity, a second message inresponse to the first message; monitoring, by the first network entity,for a failure in the P-CSCF; detecting, by the first network entity, thefailure in the P-CSCF while monitoring; and in response to detecting thefailure in the P-CSCF by the first network entity, transmitting, by thefirst network entity to a plurality of UEs associated with the P-CSCF,information for initiating an initial IMS registration for a new P-CSCF.21. The method of claim 20, further comprising transmitting informationon an address of the new P-CSCF to at least one of the plurality of UEs,based on the DHCP.
 22. The method of claim 21, wherein the address ofthe new P-CSCF is used for the initial IMS registration for the newP-CSCF.
 23. The method of claim 20, further comprising receiving a newDHCP query requesting an address of the new P-CSCF from at least one ofthe plurality of UEs.
 24. The method of claim 23, wherein the address ofthe new P-CSCF is used for the initial IMS registration for the newP-CSCF.